The present invention relates to a gate valve for use in conduits or pipelines having large nominal diameters, particularly large size hot air conduits. More particularly, the present invention relates to such a gate valve of the type having a valve casing through which cooling water is circulated and which is formed by mounting flanges attachable to the conduit and a pair of substantially U-shaped sealing rings, with inner legs of the sealing rings forming sealing surfaces for a valve plate which is mounted in the valve casing for sliding movement relative thereto between open and closed positions of the conduit.
Gate valves of this type operate to selectively close or open conduits through which gases flow at high operating temperatures. Accordingly, it is necessary to provide the interior of the conduit and the valve casing with an inner refractory brickwork lining. As a result of the high temperature stresses involved, the thickness of such brickwork lining in modern equipment is often from 300 to 500 mm. Due to this thickness, there is a large distance in the radial direction between the sealing surfaces of the sealing rings of the valve casing and the support surfaces of the mounting flanges of the valve casing. Due to the high speed flow through the large size conduits involved, when the valve plate is in the closed position, the valve plate is under a very considerable pressure, and this pressure is transferred to the sealing surfaces, thereby imparting a substantial stress thereto. This stress must be absorbed by the valve casing, and specifically, by the mounting flanges thereof. Due to the large distance in the radial direction between the sealing surfaces and the mounting flanges, the stress imparted to the sealing surfaces creates a large lever arm within the valve casing, with the result that the valve casing is stressed with large bending moments in the region of the support surfaces of the mounting flanges.
In order to overcome this disadvantage, and at the same time, to achieve sufficient stability of the valve casing, heretofore known gate valves of this type have been provided with gusset plates to absorb axial forces imparted to the sealing surfaces and to transfer such axial forces to the mounting flanges. Such gusset plates have been positioned to extend in the radial direction from the mounting flanges and the outer edge of the valve casing through an interior space within the valve casing and through which cooling water circulates to the sealing ring. In the axial direction, such gusset plates occupy almost the entire cross-section of this interior space through which cooling water is to flow. The construction of such known gate valves is however difficult since it is a very costly procedure to accurately adjust and attach the gusset plates within the interior space and then enclose the interior space with necessary cover plates. In addition, the gusset plates increase the pressure loss of the cooling water flowing through the interior space of the valve casing, tend to promote the deposition of dirt and other deposits, and have an unfavorable effect on the flow of the cooling water, since strong eddy currents occur at the gusset plates, with the resultant formation of dead zones through which the cooling water does not continuously flow.
Attempts to reinforce the valve casing by the use of thicker walls or by steel gusset plates applied inside the refractory lining have the disadvantage that the valve casing is then no longer readily expandable. This results in increased stresses within the construction of the valve casing as a result of temperature differences between the water-cooled inner surfaces and the outer surfaces which are exposed to the hot gas. Such stresses can in practice lead to cracks in the valve casing, and therefore to premature breakdown of the valve.